连接查询

外连接查询 

select ^^^^^^^^^from table1 [left|right|full] join table2 on conditions
 
例： 左外： select teacher_id, name, department_name from teachers t left outer join departments d on t.department_id = d.department_id

 

9i的版本

 右连接：select teacher_id, name, department_name from teachers t, departments d where t.department_id(+) = d.department_id

 

左外：select teacher_id, name, department_name from teachers t, departments d where t.department_id= d.department_id(+)

 

全外连接通过union来实现   即：全外 = 左外 union 右外

 

交叉连接： 可得n*m条记录

 

 select teacher_id, name, department_name from teachers, departments;

自连接  就是特殊情况两个表相同的情况，用法一样